An antiferroelectric liquid crystal is stable in an antiferroelectric state when left in a condition that no voltage (zero) is applied to the liquid crystal. Hereinafter, this stable state will be referred to as a neutral state. An antiferroelectric liquid crystal panel may be configured in such a manner as to effect either a dark display or a bright display in this neutral state. Although antiferroelectric liquid crystal panels of the present invention be applied to both a dark display and a bright display, an antiferroelectric liquid crystal panel which is adapted to effect a dark display in the neutral state will be described hereinbelow.
FIG. 7 is an example of a graph illustrating the optical transmittance of an antiferroelectric liquid crystal relative to a voltage applied thereto. In this graph, the axis of abscissa represents the applied voltages; and the axis of ordinates represents the optical transmittances.
When applying a positive voltage to the crystal, which has been in the neutral state at a point O, and increasing the positive voltage, the transmittance abruptly increases at a voltage Ft. Then, the transmittance reaches nearly the maximum value at a voltage Fs and the crystal is put into a saturated ferroelectric state. Thence, the optical transmittance does not change much even when a higher voltage is applied thereto. Next, when the applied voltage is gradually decreased, the optical transmittance abruptly drops at a voltage At. Further, the transmittance nearly reaches zero at the voltage As, and the crystal returns to an antiferroelectric state. Similarly, if a negative voltage is applied to the crystal from the voltage 0, the transmittance abruptly rises at a voltage (-Ft). Then, the transmittance nearly reaches the maximum value at a voltage (-Fs), and the crystal is put into a saturated ferrorelectric state. Thence, when the applied negative voltage is gradually reduced to 0 V, the transmittance abruptly drops at a voltage (-At). Further, the transmittance becomes almost zero at a voltage (-As), and the crystal returns to the antiferroelectric state. As above described, there are two ferroelectric states of the liquid crystal. Namely, one is the application of the positive voltage, and the other is the application of the negative voltage. Hereunder, the ferroelectric state due to the former case will be referred to as (+) ferroelectric state, while the ferroelectric state due to the latter case will be referred to as (-) ferroelectric state. Further, .vertline.Ft.vertline. designates a ferroelectric threshold voltage; .vertline.Fs.vertline. a ferroelectric saturation voltage; .vertline.At.vertline. designates an antiferroelectric threshold voltage; and .vertline.As.vertline. an antiferroelectric saturation voltage.
Generally, it is the case that the curves (namely, hysteresis curves) of FIG. 7 representing the optical transmittance characteristics of a liquid crystal relative to the voltage applied thereto are obtained by applying thereto a triangular-wave-like voltage in which the absolute value of the ratio of a change in this voltage relative to time, namely, the value of .vertline.dV/dt.vertline. is constant. However, in this case, if the value of .vertline.dV/dt.vertline. is changed, the shapes of the hysteresis curves also change. Moreover, the values of the aforementioned values As, Ft, Fs and At also vary. It is, accordingly, necessary to specify these values to specify the aforesaid value of .vertline.dV/dt.vertline.. However, the inventor obtained FIG. 7 by the following method (hereunder referred to as a time fixation method 1) so as to obtain values more corresponding to actual driving conditions.
It is assumed that the duration of one frame (to be described later) of a display device to be used in a working temperature, is Pt and that the length of a time period, in which a selection voltage (to be described later) is applied, is Wt.
(1) A pulse voltage, whose duration is Wt and voltage level is Vx, is applied to the liquid crystal that is in a stable antiferroelectric state (namely, in the neutral state). Further, the relationship between the optical transmittance and the pulse voltage Vx at the time of completion of the application of this pulse voltage is plotted. Moreover, this operation is repeated by changing the value of the voltage Vx. Thereby, the curve drawn from the point O to Fs through Ft of FIG. 7, as well as the curve drawn from the point O to (-Fs) through the (-Ft), is obtained.
(2) Next, the liquid crystal is first put into the saturated ferroelectric state by applying thereto a voltage which is not lower than the aforementioned voltage .vertline.Fs.vertline.. Then, at a moment 0, the applied voltage is reduced to .vertline.Vz.vertline.. Thence, after the elapse of the assumed relaxation period (to be described later), the relationship between the value of the optical transmittance and the applied voltage Vz is plotted. This operation is repeated by changing the value of the voltage .vertline.vz.vertline.. Thereby, the curve drawn from Fs to the point O through At and As of FIG. 7, as well as the curve drawn from (-Fs) to the point O through (-At) and (-As), is obtained.
When some liquid crystal panels are used, the curve (namely, the curve drawn from Fs or (-Fs) to the point O in FIG. 7) obtained in the aforementioned case (2) sometimes intersects the ordinate axis. The main cause of this is the responsivity of the liquid crystal. Namely, in the case that the liquid crystal is maintained in the ferroelectric state by applying thereto a voltage, which is not lower than the aforementioned voltage .vertline.Fs.vertline., and that at the moment 0, the applied voltage Vz is changed into 0, the liquid crystal finally becomes stable in the antiferroelectric state after the elapse of a certain time period (hereunder referred to as a relaxation time tn). However, if this relaxation time tn is longer than the relaxation period (to be described later), the curve obtained in the aforementioned case (2) intersects the ordinate axis.
When actually driven, it is difficult to bring such a liquid crystal panel into a complete antiferroelectric state, and a dark display cannot be effected and that the contrast is extremely degraded.
Generally, a liquid crystal panel is driven by performing the following process. Namely, first, N row electrodes and M column electrodes are formed in such a manner as to be arranged as a matrix of N rows and M columns. Further, a scanning signal is applied to each of the row electrodes through a row-electrode drive circuit, while a display signal depending on display data of each pixel (incidentally, a part of the display signal is sometimes not dependent on the display data) is applied to each of the column electrodes through a column-electrode drive circuit. Moreover, a voltage (hereunder referred to simply as a synthesis voltage), which corresponds to the difference between the scanning signal and the display signal, is applied to a liquid crystal layer. The time period required to scan all of the row electrodes (namely, 1 vertical scanning interval) is usually referred to as 1 frame (or 1 field). In the case of driving the liquid crystal panel, the polarity of a driving voltage is reversed or inverted each frame (or every frames) in order to prevent the liquid crystal from being adversely affected (for example, the degradation due to non-uniform distribution of ions).
FIG. 9 illustrates the waveforms of signals flowing through the row electrodes, the column electrodes and the pixel synthesis electrodes of a liquid crystal panel in which the N row electrodes and the M column electrodes are formed in such a manner as to be arranged as a matrix of N rows and M columns. The display conditions or states of pixels are assumed as follows. Namely, in the case of a first column (Y1), pixels in all rows are displayed in white. Further, in the case of a second column (Y2), a pixel in a first row is displayed in black, and pixels in the other rows are displayed in white. Moreover, in the case of pixels in a third column (Y3), these pixels are displayed alternately in black and in white. Furthermore, in the case of an Mth column, Ym pixels are displayed in black in all rows.
Waveforms of scanning signals are respectively applied to the N row electrodes in sequence from the top row to the bottom row so that each of the waveforms is shifted by a time (1/N). Waveforms of display signals applied to the M column electrodes are synchronized with the scanning signal and the waveforms according to whether the pixels are displayed in white or in black are applied.
Paying attention to a synthesis voltage applied to each pixel, with respect to P11 displayed in white and P12 displayed in black in the first row, the voltage applied to P11 in the selection period tw, which is displayed in white, is a large waveform, whereas the voltage applied to P12 in the selection period tw, which is displayed in black is a small waveform. The synthesis voltage applied to a pixel P21, which is displayed in white in a second row, has a waveform which is almost the same as obtained by shifting the waveform of the synthesis voltage applied to the pixel P11 by (1/N). Here, note that the first and the second frame in the first row and the second row are shifted with respect to each other by (1/N).
Turning attention to the scanning signal to be applied to a single row electrode, 1 vertical scanning interval is composed of N horizontal scanning intervals (in some case, an additional interval is added thereto). Among a horizontal scanning interval, a part of horizontal scanning interval in which a scanning voltage (hereunder referred to as the selection voltage) to be used for determining the display condition of a pixel on this row is applied, is referred to as a selection period tw. The other part of horizontal scanning interval are referred to as non-selection periods.
Usually, in the case of the antiferroelectric liquid crystal panel, it is determined on the basis of the aforementioned display signal at the time of applying the selection voltage whether the state of the liquid crystal, which has been in the antiferroelectric state, is maintained or is changed into the ferroelectric state. Thus, there is the necessity of a time period (hereunder referred to as a relaxation period ts) required for setting the liquid crystals in the antiferroelectric state before the application of the selection voltage. During a time period which is other than the selection period tw and the relaxation period ts, the determined state of the liquid crystal should be held. Hereunder, this time period will be referred to as a holding period tk.
FIG. 10 illustrates the waveforms of a scanning signal waveform (Pa) applied to a given pixel of interest, display signal waveforms (Pb, Pb'), synthetic signal waveforms (Pc, Pc') and optical transmittances L100 and L0 according to the driving method described in FIGS. 1 and 2 of the Japanese Unexamined Patent Publication (Kokai) No. 4-362990/1992.
In FIG. 10, F1 and F2 designate the first frame and the second frame, respectively. This figure illustrates the case where the polarity of the aforementioned driving voltage is inverted every frame. As is apparent from this figure, the first frame F1 is different from the second frame F2 only in that the polarity of the driving voltage is inverted. As is obvious from the aforementioned FIG. 7, an operation of the liquid crystal display device is symmetric with respect to the polarity of the driving voltage. Therefore, the following description will be given regarding only the first frame, except in case of necessity.
Further, in the following description and drawings of the waveform of driving signals, the electric potential indicated as "0" does not mean absolute electric potential but means the reference electric potential. Therefore, in the case that the reference electric potential varies for some reason, scanning signals and display signals vary relatively. Moreover, in the case that the word "voltage" is used in connection with the scanning signals and the display signals in the following description, the word "voltage" designates the difference between the electric potential indicated by such a signal and the reference electric potential.
Incidentally, the value of each of the aforementioned ferroelectric threshold voltage .vertline.Ft.vertline., the aforementioned ferroelectric saturation voltage .vertline.Fs.vertline., the aforementioned antiferroelectric threshold voltage .vertline.At.vertline. and the aforementioned antiferroelectric saturation voltage .vertline.As.vertline. in the (+) ferroelectric side is sometimes slightly different from the value thereof in the (-) ferroelectric side. However, for simplicity of description, the following description will be presented by assuming that each of these voltage has the same value.
As shown in FIG. 10, 1 frame is divided into three time periods, namely, the selection period tw, the holding period tk and the relaxation period ts. The selection period tw is further divided into time periods tw1 and tw2, which have equal lengths. The voltage level of a scanning signal Pa in the first frame F1 is set as follows. Needless to say, in the second frame F2, the polarity of the voltage is inverted. Here, note that .+-.V designates the selection voltage and that the length of the time period tw2 corresponds to the aforementioned Wt.
Time Period tw1 tw2 tk ts Scanning Signal Voltage 0 +V1 +V3 0
Further, the display signal is set as follows according to the display state. Here, note that the symbol "*" indicates that the voltage depends on the display data of other pixels in a same column as this pixel.
 Time Period tw1 tw2 tk ts On-State Display Signal Voltage +V2 -V2 * * Off-State Display Signal Voltage -V2 +V2 * *
In the case of the hysteresis curves of FIG. 7, generally, the curve drawn from As to Ft or from At to Fs is not flat. Thus, when the voltage applied to the liquid crystal in the holding period tk is shifted depending on the display signal, a change in the brightness in this holding period is caused. To prevent an occurrence of this phenomenon, usually, the polarity of the display signal is inverted in such a manner that the average value thereof in a horizontal scanning interval is 0. Namely, the polarity of the display signal in the time period tw1 is changed in the time period tw2.
In FIG. 10, Pb, Pc and L100 respectively denote the waveform of a display signal, that of a synthetic signal and optical transmittance in the case that all of the pixels provided on a column electrode, to which a pixel of interest belongs, are in an on-state (a bright state). In this case, if the (synthetic) voltage to be applied to the liquid crystal in the time period tw2 meets the following condition: .vertline.V1+V2.vertline.&gt;.vertline.Ft.vertline. (see FIG. 7), the transition of the state of the liquid crystal into the ferroelectric state is started. As a result, the optical transmittance of the liquid crystal increases. In the holding period tk, if the following condition is satisfied: .vertline.V3-V2.vertline.&gt;.vertline.At.vertline., the bright state is held. In the relaxation period ts, if the following condition is satisfied: .vertline.V2.vertline.&lt;.vertline.As.vertline., the transmittance decreases with the elapse of time. Thus, the relaxation of the liquid crystal, namely, the change of the state thereof from the ferroelectric state to the stable antiferroelectric state is expected to occur.
Further, in FIG. 10, Pb', Pc' and L0 respectively designate the waveform of a display signal, that of a synthetic signal and optical transmittance in the case that all of the pixels provided on a column electrode, to which a pixel of interest belongs, are in an off-state (a dark state). In this case, if the synthetic voltage to be applied to the liquid crystal in the time period tw2 meets the following condition: .vertline.V1-V2.vertline.&lt;.vertline.Ft.vertline., the voltage applied in the holding period tk meets the following condition: .vertline.V3+V2.vertline.&lt;.vertline.Ft.vertline., and the voltage applied in the relaxation period ts meets the following condition: .vertline.V2.vertline.&lt;.vertline.Ft.vertline., it can be expected that the dark state is caused.
It is, however, found that as indicated by dashed line in the dark state L0 of FIG. 10, even if the voltage applied during the holding period tk meets the condition: .vertline.V3+V2.vertline.&lt;.vertline.Ft.vertline., the mean value of the optical transmittance gradually increases and thus, black display become unable to be presented, and that consequently, the contrast is sometimes degraded. It is further known that this phenomenon occurs in the case when the voltage applied in the time period tw2 has a value between .vertline.Ft.vertline. and .vertline.Fs.vertline., namely, in the case when halftone gray scale are displayed. Therefore, it is found that this phenomenon results not only in deterioration in the contrast but also in gradual increase in the mean value of the optical transmittance during the holding period tk even in the case of displaying halftone gray scale and that there is caused a serious problem in that a linear-gray shades display cannot be obtained.